The present invention provides a process for preparing a methyl ester by reacting a carboxylic acid or salt thereof with dimethyl carbonate in the presence of a catalyst selected from 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), and 1,4-diazabicyclo[2.2.2]octane (DABCO).
Methylation of alcohols, amines, carboxylic acids and activated methylenes is an important process in chemistry. However, due to the environmental and human impact of using toxic and unsafe methylating reagents, such as methyl iodide or dimethyl sulfate, the investigation of safer, generally applicable alternatives continues. As an alternative to these toxic methylating agents, dimethyl carbonate has attracted considerable attention for the methylation of phenols, anilines and activated methylenes. Dimethyl carbonate is non-toxic and generates CO2 and methanol as by-products during methylations. Dimethyl carbonate is also a volatile liquid with a boiling point of 90xc2x0 C. Hence, the unreacted dimethyl carbonate can be easily recovered by distillation from the reaction mixture and reused. Furthermore, dimethyl carbonate has been shown to be quite selective in monomethylation of primary aromatic amines and C-methylation of arylacetonitriles and arylacetoesters.
However, the use of dimethyl carbonate as a methylating regent requires high temperatures and pressures, generally 140xc2x0 C.-180xc2x0 C. Therefore, autoclaves or the use of asymmetrical carbonates with a higher boiling point than dimethyl carbonate have to be employed. These restrictions lower the popularity of using dimethyl carbonate as a methylating reagent.
U.S. Pat. No. 4,513,146 describes a method for producing esters from highly-hindered carboxylic acids and carbonates. The method involves reacting the highly-hindered carboxylic acid with a carbonate with or without a catalyst at a temperature of 175xc2x0 C. according to the examples. U.S. Pat. No. 4,513,146 states that exemplary catalysts are nitrogen-containing heterocyclic catalysts such as pyridine, 4-(dimethylamino)pyridine, imidazole, 2,6-lutidine and 2,4,6-collidine.
Therefore, it would be advantageous from a production and safety standpoint to develop a low temperature process which utilizes dimethyl carbonate as a reactant in the production of esters.
The invention provides a low temperature process for preparing a methyl ester having formula (III) 
said process comprising reacting a carboxylic acid or salt thereof having formula (I) 
with dimethyl carbonate having formula (II) 
in the presence of a catalyst selected from the group consisting of 1,8-diazabicyclo[5.4.0]undec-7-ene; 1,4-diazabicyclo[2.2.2]octane; and combinations thereof, wherein said process is conducted at a temperature of about 10xc2x0 C. to less than 120xc2x0 C.;
R1 is selected from the group consisting of an alkyl, aryl, alkoxy, alkenyl, cycloalkyl, benzocycloalkyl, cycloalkylalkyl, aralkyl, heterocyclic, heteroaralkyl, alkoxyalkyl, carboxyalkyl, alkylcarbonyl, alkoxycarbonyl, alkoxycarbonylalkyl and haloalkyl; and M is selected from the group consisting of hydrogen, a monovalent metal and a monovalent fractional part of a polyvalent metal.
According to another aspect, the invention provides a compound having formula (III) 
wherein said compound is prepared by a process comprising reacting a carboxylic acid or salt thereof having formula (I) 
with dimethyl carbonate having formula (II) 
in the presence of a catalyst selected from the group consisting of 1,8-diazabicyclo[5.4.0]undec-7-ene; 1,4-diazabicyclo[2.2.2]octane; and combinations thereof, wherein said process is conducted at a temperature of about 10xc2x0 C. to less than 120xc2x0 C.;
R1 is selected from the group consisting of an alkyl, aryl, alkoxy, alkenyl, cycloalkyl, benzocycloalkyl, cycloalkylalkyl, aralkyl, heterocyclic, heteroaralkyl, alkoxyalkyl, carboxyalkyl, alkylcarbonyl, alkoxycarbonyl, alkoxycarbonylalkyl and haloalkyl; and M is selected from the group consisting of hydrogen, a monovalent metal and a monovalent fractional part of a polyvalent metal.
The process of the invention is especially advantageous for preparing methyl esters since the process: (1) is conducted at a low temperature, less than 120xc2x0 C.; (2) utilizes an environmentally friendly methylating reagent, dimethylcarbonate; (3) produces a high yield of the methyl ester, generally 97-98% conversion; and (4) does not require a high-pressure (autoclave) reactor.
The process of the invention is used to prepare a methyl ester having formula (III) 
In formula (III), R1 is selected from the group consisting of an alkyl, aryl, alkoxy, alkenyl, cycloalkyl, benzocycloalkyl, cycloalkylalkyl, aralkyl, heterocyclic, heteroaralkyl, alkoxyalkyl, carboxyalkyl, alkylcarbonyl, alkoxycarbonyl, alkoxycarbonylalkyl and haloalkyl. The reaction is conducted in the presence of a catalyst which is selected from 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and 1,4-diazabicyclo[2.2.2]octane (DABCO). A combination of catalysts may also be used.
The process of the invention involves reacting a carboxylic acid or salt thereof having formula (I) 
with dimethyl carbonate having formula (II) 
In formula (I), R1 is selected from the group consisting of an alkyl, aryl, alkoxy, alkenyl, cycloalkyl, benzocycloalkyl, cycloalkylalkyl, aralkyl, heterocyclic, heteroaralkyl, alkoxyalkyl, carboxyalkyl, alkylcarbonyl, alkoxycarbonyl, alkoxycarbonylalkyl and haloalkyl, and M is selected from the group consisting of hydrogen, a monovalent metal and a monovalent fractional part of a polyvalent metal. The metal is preferably selected from sodium, potassium, magnesium or calcium.
As used herein, xe2x80x9calkylxe2x80x9d means straight chain or branched alkyl, which may be, for example, C1-C10-alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, straight or branched pentyl, straight or branched hexyl, straight or branched heptyl, straight or branched nonyl or straight or branched decyl. Preferably alkyl is C1-C4-alkyl. xe2x80x9cArylxe2x80x9d means C6-C14-aryl, preferably C6-C10-aryl, and may be, for example, substituted by at least one group selected from mercapto, dialkylamino, nitro, alkoxy, halogen, keto, cyano or a combination. Preferably aryl is phenyl.
xe2x80x9cAlkoxyxe2x80x9d means straight chain or branched alkoxy and may be, for example, C1-C10-alkoxy, such as methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, isobutoxy, sec-butoxy, tert-butoxy or straight or branched pentoxy, hexyloxy, heptyloxy, octyloxy, nonyloxy or decyloxy. Preferably alkoxy is C1-C4-alkoxy.
xe2x80x9cAlkenylxe2x80x9d means straight chain or branched alkenyl, which may be, for example, C2-C10 alkenyl, such as vinyl, 1-propenyl, 2-propenyl, 1-butenyl, isobutenyl, or straight or branched pentenyl, hexenyl, heptenyl, octenyl, nonenyl or decenyl. Preferred alkenyl is C2-C4-alkenyl.
xe2x80x9cCycloalkylxe2x80x9d means C3-C10-cycloalkyl having 3- to 8-ring carbon atoms and may be, for example, cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl or cycloheptyl, any of which can be substituted by one, two or more C1-C4-alkyl groups, particularly methyl groups. Preferably, cycloalkyl is C3-C6-cycloalkyl.
xe2x80x9cBenzocycloalkylxe2x80x9d means cycloalkyl (e.g., one of the C3-C10-cycloalkyl groups mentioned hereinbefore), attached at two adjacent carbon atoms to a benzene ring. Preferably, benzocycloalkyl is benzo-C5-C6-cycloalkyl, especially benzocyclohexyl (tetrahydronaphthyl).
xe2x80x9cCycloalkylalkylxe2x80x9d means C3-C10-cycloalkyl-C1-C10-alkyl, where the C3-C10-cycloalkyl group has 3- to 8-ring carbon atoms and may be, for example, one of the C1-C10-alkyl groups mentioned hereinbefore, particularly one of the C1-C4-alkyl groups, substituted by one of the C3-C10-cycloalkyl groups mentioned hereinbefore. Preferably cycloalkylalkyl is C3-C6-cycloalkyl-C1-C4-alkyl.
xe2x80x9cAralkylxe2x80x9d means straight chain or branched C6-C10-aryl-C1-C10-alkyl and may be, for example, one of the C1-C10-alkyl groups mentioned hereinbefore, particularly one of the C1-C4-alkyl groups, substituted by phenyl, tolyl, xylyl or naphthyl. Preferably, aralkyl is phenyl-C1-C4-alkyl, particularly benzyl or 2-phenylethyl.
xe2x80x9cHeterocyclicxe2x80x9d means a monovalent heterocyclic group having up to 20 carbon atoms and one, two, three or four heteroatoms selected from nitrogen, oxygen and sulfur, the group optionally having an alkyl, alkylcarbonyl, hydroxyalkyl, alkoxyalkyl or aralkyl group attached to a ring carbon or nitrogen atom and being linked to the remainder of the molecule through a ring carbon atom, and may be, for example, a group, preferably a monocyclic group, with one nitrogen, oxygen or sulfur atom such as pyrryl, pyridyl, piperidyl, furyl, tetrahydrofuryl or thienyl, or a group, preferably a monocyclic group, with two hetero atoms selected from nitrogen, oxygen and sulfur, such as imidazolyl, pyrimidinyl, piperazinyl, oxazolyl, isoxazolyl, thiazolyl, morpholinyl or thiomorpholinyl. Preferably, heterocyclic is a monocyclic group having 5- or 6-ring atoms and one or two nitrogen atoms, or one nitrogen atom and one oxygen atom, in the ring and optionally substituted on a ring nitrogen atom by C1-C4-alkyl, hydroxy-C1-C4-alkyl, C1-C4-alkylcarbonyl or phenyl-C1-C4-alkyl.
xe2x80x9cHeteroaralkylxe2x80x9d means straight chain or branched aralkyl (e.g., one of the C6-C10-aryl-C1-C10-alkyl groups mentioned hereinbefore) substituted by one or more heterocyclic groups.
xe2x80x9cAlkoxyalkylxe2x80x9d means straight chain or branched alkyl substituted by one or more alkoxy groups and may be, for example, a C1-C10-alkoxy-C1-C10-alkyl group, such as one of the C1-C10-alkyl groups, particularly one of the C1-C4-alkyl groups, mentioned hereinbefore substituted by one of the C1-C10-alkoxy groups, preferably one of the C1-C4-alkoxy groups mentioned hereinbefore. Preferably alkoxyalkyl is C1-C4-alkoxy-C1-C4-alkyl.
xe2x80x9cCarboxyalkylxe2x80x9d means straight chain or branched alkyl, for example, C1-C10-alkyl, such as one of the C1-C10-alkyl groups mentioned hereinbefore, substituted, preferably on a primary carbon atom, by a carboxyl group. Preferably carboxyalkyl is carboxy-C1-C4-alkyl.
xe2x80x9cAlkylcarbonylxe2x80x9d means a group R2Cxe2x95x90O wherein R2 is alkyl, for example, C1-C10-alkyl, such as one of the C1-C10-, preferably C1-C4-, alkyl groups mentioned hereinbefore. Preferably, alkylcarbonyl is C1-C4-alkylcarbonyl, for example, R2Cxe2x95x90O wherein R2 is C1-C4-alkyl.
xe2x80x9cAlkoxycarbonylxe2x80x9d means a group R3CO wherein R3 is an alkoxy group, for example, a C1-C10-alkoxy group, such as one of the C1-C10, preferably C1-C4, alkoxy groups mentioned hereinbefore. Preferably, alkoxycarbonyl is C1-C4-alkoxycarbonyl, for example, R3CO, wherein R3 is C1-C4-alkoxy.
xe2x80x9cAlkoxycarbonylalkylxe2x80x9d means straight or branched chain alkyl, for example, a C1-C10-alkyl group, such as one of the C1-C10-, preferably C1-C4-, alkyl groups mentioned hereinbefore, substituted by an alkoxycarbonyl group as hereinbefore defined. Preferably, alkoxycarbonylalkyl is C1-C4-alkoxy-carbonyl-C1-C4-alkyl.
xe2x80x9cHaloalkylxe2x80x9d means straight chain or branched alkyl, for example, C1-C10-alkyl, such as one of the C1-C10-alkyl groups mentioned hereinbefore, substituted by one or more, for example, one, two or three halogen atoms, preferably fluorine or chlorine atoms. Preferably haloalkyl is C1-C4-alkyl substituted by one, two or three fluorine or chlorine atoms.
Specific examples of carboxylic acids of formula (I) are 2,6-dimethoxybenzoic acid, 2,3:4,6-di-O-isopropylidene-xcex1-L-xylo-2-hexulofuranosonate monohydrate, N-xcex1-t-boc-L-proline, benzoic acid, phthalic acid, isophthalic acid, naphthalene-2,6-dicarboxylic acid, cyclohexanedicarboxylic acid, cyclohexanediacetic acid, diphenyl-4,4xe2x80x2-dicarboxylic acid, succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, 2,2-dimethylpropanoic acid, 2,2-dimethylbutanoic acid, 2,2-dimethyloctanoic acid and the sodium and potassium salts thereof. A combination of carboxylic acids may also be used. Preferably the carboxylic acid is selected from 2,6-dimethoxybenzoic acid, 2,3:4,6-di-O-isopropylidene-xcex1-L-xylo-2-hexulofuranosonate monohydrate, N-xcex1-t-boc-L-proline, and benzoic acid.
The process of the invention is preferably conducted in the liquid phase. It may be carried out batchwise, continuously, semi-batchwise or semi-continuously. When the dimethyl carbonate is a liquid under the conditions of the reaction, it may act as a solvent for the carboxylic acid or salt thereof. Typically, but not necessarily, excess dimethyl carbonate is employed relative to the amount of carboxylic acid or salt thereof, and this usually serves to dissolve the carboxylic acid or salt thereof throughout the reaction. In many cases, one or more by-products of the reaction, most notably methanol, also tend to dissolve the carboxylic acid or salt thereof. Although extrinsic solvent is not ordinarily employed, it may be used when desired or when necessary to dissolve one or more of the reactants. Examples of suitable extrinsic solvents include: acetonitrile, ethyl acetate, acetone, benzene, toluene, dioxane, dimethylformamide and chlorinated solvents, such as chloroform, methylene chloride, ethylene chloride, carbon tetrachloride and chlorobenzene. A combination of solvents may also be used. Preferably, the process is conducted without an extrinsic solvent.
The process of the invention is conducted at a temperature of from about 10xc2x0 C. to less than 120xc2x0 C., preferably, 70xc2x0 C.-100xc2x0 C.; and more preferably from about 90xc2x0 C. to less than 100xc2x0 C. The process is conducted preferably under a pressure of from about 1 atm to about 100 atm, more preferably, from 1 atm to 50 atm. Most preferably, the process is conducted under a pressure of 1 atm.
The equivalent ratio of the catalyst to the carboxylic acid or salt thereof initially present may vary widely, but preferably is in the range of from about 0.01:1 to about 2:1. More preferably, the equivalent ratio of the catalyst to the carboxylic acid or salt thereof initially present is from about 0.1:1 to about 1:1, most preferably, about 1:1.
In one embodiment of the invention, an amine base is used in the process of the invention to prepare a methyl ester. Preferred amine bases are trialkylamines and ethylenediamines. Specific amine bases include, but are not limited to, triethylamine, N,N-diisopropylethylamine and N,Nxe2x80x2-diisopropylethylenediamine. A combination of amine bases may also be used.
Following preparation, the methyl ester may be recovered from the reaction mixture by any of the various techniques known to the art. Distillation at reduced pressure is a preferred technique.
The following non-limiting examples illustrate further aspects of the invention.